Surface modification of graphite fibers

ABSTRACT

Graphite fibers exhibiting an enhanced ability to bond to a matrix material (e.g., a thermosetting resinous material or a metallic material) are produced wherein a film of amorphous carbon in intimate association with titanium is provided upon the surface of the same. A predominantly graphitic carbonaceous fibrous material is coated with a film of a dihydropyridacene polymer in intimate association with a hydrolyzable organotitanium compound, the organotitanium compound hydrolyzed to form titanium dioxide, and the polymer portion of the film carbonized to a predominantly amorphous form. Composite articles which incorporate the carbon fibers produced by the present process exhibit improved physical properties, such as an improved interlaminar shear strength.

United States Patent [191 Daley et al,

[111 3,821,013 1 June 28, 1974 SURFACE MODIFICATION OF GRAPHITE FIBERSInventors: Lawrence R. Daley; George R.

Ferment, both of Dover, NJ.

Assignee: Celanese Corporation, New York,

Filed: Feb. 7,1972

Appl. No.: 223,975

11.8. CI. 117/46 CC, 8/1156, 117/169 R, 117/226, 117/228, 423/447,264/D1G. 19, 1l7/DIG. 11

Int. Cl 344d 5/12 Fieldof Search 117/46 CB, 46 CC, 169 R, 117/226, 160R, 161 ZB, 228, DIG. 11; 423/447;8/1l5.6; 264/29, DIG. 19

References Cited UNITED STATES PATENTS 1/1973 Leeds 117/46 CC PrimaryExaminer-Wi11iam D. Martin Assistant Examiner-Janyce A. Bell Attorney,Agent, or Firm-Thomas J. Morgan [57] ABSTRACT dioxide, and the polymerportion of the film carbon ized to a predominantly amorphous form.Composite articles which incorporatethe carbon fibers produced by thepresent process exhibit improved physical propll erties, such as animproved interlammar shear strength.

14 Claims, 2 Drawing Figures 1 SURFACE MODIFICATION OF GRAPHITE FIBERSBACKGROUND OF THE INVENTION In the search for high performancematerials, considerable interest has been focused upon graphitic carbonfibers. Graphite fibers are defined herein as fibers which consistsubstantially of carbon and have a pre dominant x-ray, diffractionpattern characteristic of graphite. Amorphous carbon fibers, on theother hand,

are defined as fibers in which the bulk of the fiber I weight can beattributed to carbon and which exhibit a predominantly amorphous x-raydiffraction pattern.

Graphite fibers generally have a higher Youngs modulus than do amorphouscarbon fibers and in addition are more electrically and thermallyconductive.

Industrial high performance materials of the future are projected tomake substantial utilization of fiber reinforced composites, andgraphitic carbon fibers theoretically have among the best properties ofany fiber for use as high strength reinforcement. Among these desirableproperties are corrosion and high temperature re sistance, low density,high tensile strength, and high modulus. Graphite is one of the very fewknown materials whose tensile strength increases with temperature.

'Uses for graphitic carbon fiber reinforced composites include aerospacestructural components, rocket motor casings, deep-submergence vesselsand ablative materials for heat shields on re-entry vehicles.

In the prior art numerous materials have been proposed for use aspossible matrices in which graphitic carbon fibers may be incorporatedto provide reinforcement and produce a composite article. The matrixmaterial which is selected is commonly a thermosetting resinous materialand is commonly selected because of its ability to also withstand highlyelevated temperatures. Metallic matrix materials may also be utilized.

While it has been possible in the past to provide graphitic carbonfibers of highly desirable strength and modulus characteristics,difficulties have arisen when one attempts to gain the full advantagesof such properties in the resulting carbon fiber reinforced compositearticle; Such inability to capitalize upon the superior single filamentproperties of the reinforcing fiber has been traced to inadequateadhesion between the fiber and the matrix in the resulting compositearticle.

Various techniques have been proposed in the past for modifying thefiber properties of a previously formed carbon fiber in order to makepossible improved adhesion when present in a composite article. See, forinstance, US. Pat. No. 3,476,703 and British Pat. No. 1,180,441 toNicholas J. Wadsworth and William Watt wherein it is taught to heat acarbon fiber normally within the range of 350C. to 850C. (e.g. 500 to600C.) in an oxidizing atmosphere such as air for an appreciable periodof time. Other atmospheres contemplated for use in the process includean oxygen rich atmosphere, pure oxygen, or an atmosphere containing anoxide of nitrogen from which free oxygen becomes available such asnitrous oxide and nitrogen dioxide. Improved carbon fiber surfacemodification processes are disclosed in commonly assigned U.S. Ser. Nos.65,454 (now US. Pat. No. 3,734,957) and 65,456, (now U.S. Pat. No.3,732,150), filed Aug. 20, 1970; and U5. Ser. No. 99,169 (now US. Pat.No. 3,745,104), filed Dec. 17, 1970.

It is an object of the invention to provide a continuous process formodifying the surface characteristics of graphitic carbon fiberspossessing an enhanced ability to bond to a matrix material.

It is an object of the invention to provide graphitic carbon fiberspossessing modified surface characteristics which eliminates the needfor heating the same in an oxidizing atmosphere as commonly conducted inthe prior art.

It is another object of the invention to provide a process for producingcarbon fibers possessing an enhanced ability to bond to a matrixmaterial without degradation of the graphitic carbon fiber tensileproperties.

It is another object of the invention to provide composite articlesreinforced with graphitic carbon fibers exhibiting an improvedinterlaminar shear strength.

These and other objects, as well as the scope, nature, and utilizationof the invention will be apparent from the following detaileddescription and appended claims.

SUMMARY OF THE INVENTION It has been found that a process for thesurface modification of a predominantly graphitic carbonaceous fibrousmaterial containing at least 90 per cent carbon by weight comprises: i

a. coating the fibrous material with a film of a dihydropyridacenepolymer which is substantially free of inter-molecular cross-linkingconsisting of to 100 mol per cent of acrylonitrile units wherein thependant nitrilegroups thereof are at least about percent cyclized, and 0to 15 mol per cent of copolymerized monovinyl units, with thedihydropyridacene polymer film being in intimate association with ahydrolyzable organotitanium compound capable of yielding titaniumdioxide upon hydrolysis,

b. hydrolyzing the hydrolyzable organotitanium compound in intimateassociation with the dihydropyridacene polymer film to form titaniumdioxide, and

c. carbonizing the dihydropyridacene polymer portion of the film presentupon the fibrous material to a predominantly amorphous carbon formcontaining at least about 90 per cent carbon by weight by heating in aninert gaseous atmosphere at a temperature of at least about 900C, butnot exceeding about 1,800C. to produce a predominantly graphiticcarbonaceous fibrous material which possesses an enhanced ability tobond to a matrix material.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a photograph made with the aid ofa scanning electron microscope of a representative graphitic carbonfiber surface modified in accordance with the present invention at amagnification of 6600X possessing an enhanced ability to bond to amatrix material.

FIG. 2 is a photograph made with the aid of a scanning electronmicroscope of a control representative graphitic carbon fiber at amagnification of 6400X which underwent no form or surface modification.

DESCRIPTION OF PREFERRED EMBODIMENTS The Starting Material Thecarbonaceous fibers which are modified in accordance with the process ofthe present invention contain at least about 90 per cent carbon byweight and exhibit a predominantly graphitic x-ray diffraction pattern.In a preferred embodiment of the process the graphitized carbonaceousfibers which undergo surface modification contain at least about 95 percent carbon by weight (e.g., at least about 99 per cent carbon byweight).

The graphitized carbonaceous fibrous materials are preferably providedin a continuous length in any one of a variety of physicalconfigurations provided substantial access to the fiber surface ispossible during the surface modification treatment described hereafter.For instance, the fibrous materials may assume the configuration of acontinuous length of a multifilament yarn, tape, tow, strand, cable, orsimilar fibrous assemblage. In a preferred embodiment of the process thefibrous material is one or more continuous multifilament yarn'or a tow.When a plurality of multifilament yarns are surface treatedsimultaneously (as described hereafter), they may be continuouslyprocessed while in parallel and in the form of a flat ribbon, as may aflat tow.

The previously graphitized carbonaceous fibrous material which istreated in the present process optionally may be provided with a twistwhich tends to improve the handling characteristics. For instance, atwist of about 0.1 to 5 tpi, and preferably about 0.3 to 1.0 tpi, may beimparted to a multifilament yam. Also, a false twist may be used insteadof or in addition to a real twist. Alternatively, one may selectcontinuous bundles of fibrous material which possess essentially notwist.

The graphitized carbonaceous fibers which serve as the starting materialin the present process may be formed in accordance with a variety oftechniques as will be apparent to those skilled in the art. Forinstance, organic polymeric fibrous materials which are capable ofundergoing thermal stabilization may be initially stabilized bytreatment in an appropriate atmosphere at a moderate temperature (e.g.200 to 400C), and subsequently heated in an inert atmosphere at a morehighly elevated temperature until a carbonized and graphitized fibrousmaterial is formed. For instance, the thermally stabilized material maybe carbonized by heating in an inert atmosphere at a temperature ofabout 900 to 1,000C. and subsequently heated to a maximum temperature of2,000 to 3,IOOC. (preferably 2,400 to 3,100C.) in an inert atmospherefor a sufficient residence time to produce substantial amounts ofgraphitic carbon.

The exact temperature and atmosphere utilized during the initialstabilization of an organic polymeric fibrous material commonly varywith the composition of the precursor as will be apparent to thoseskilled in the art. During the carbonization reaction elements presentin the fibrous material other than carbon (e.g., oxygen and hydrogen)are substantially expelled. Suitable organic polymeric fibrous materialsfrom which the graphitized carbonaceous fibrous materials may be derivedinclude an acrylic polymer, a cellulosic polymer, a polyamide, apolybenzimidazole', polyvinyl alcohol, etc. As discussed hereafter,acrylic polymeric materials are particularly suited for use asprecursors in the formation of graphitized carbonaceous fibrousmaterials. Illustrative examples of suitable cellulosic materialsinclude the natural and regenerated forms of cellulose, e.g., rayon.Illustrative examples of suitable polyamide materials include thearomatic polyamides, such as nylon 6T, which is formed by thecondensation of hexamethylenediamine and terephthalic acid. Anillustrative example of a suitable polybenzimidazole is poly-2,2-m-phenylene-5,5'-bibenzimidazole.

A fibrous acrylic polymeric material prior to stabilization may beformed primarily of recurring acrylonitrile units. For instance, theacrylic polymer should contain not less than about mol per cent ofrecurring acrylonitrile units with not more than about 15 mol per centof a monovinyl compound which is copolymerizable with acrylonitrile suchas styrene, methyl acrylate, methyl methacrylate, vinyl acetate, vinylchloride, vinylidene chloride, vinyl pyridine, and the like, or aplurality of such monovinyl compounds.

During the formation of a preferred graphitized carbonaceous fibrousmaterial for use in the present process multifilament bundles of anacrylic fibrous material may be initially stabilized in anoxygen-containing atmosphere (i.e., preoxidized) on a continuous basisin accordance with the teachings of U.S. Ser. No. 749,957, filed Aug. 5,1968, of Dagobert E. Stuetz, which is assigned to the same assignee asthe present invention and is herein incorporated by reference. Morespecifically, the acrylic fibrous material should be either anacrylonitrile homopolymer or an acrylonitrile copolymer which containsno more than about 5 mol per cent of one or more monovinyl comonomerscopolymerized with acrylonitrile. In a particularly preferred embodimentof the process the fibrous material is derived from an acylonitrilehomopolymer. The stabilized acrylic fibrous material which ispreoxidized in an oxygen-containing atmosphere is black in appearance,contains a bound oxygen content of at least about 7 per cent by weightas determined by the Unter' zaucher analysis, retains its originalfibrous configuration essentially intact, and is non-burning whensubjected to an ordinary match flame.

In preferred technique for forming the starting material for the presentprocess a stabilized acrylic fibrous material is carbonized andgraphitized while passing through a temperature gradient present in aheating zone in accordance with the procedures described in commonlyassigned U.S. ,Ser. Nos. 777,275, filed Nov. 20, 1968 (now abandoned) ofCharles M. Clarke; 17,780, filed March 9, 1970 (now U.S. Pat. No.3,677,705) of Charles M. Clarke, Michael .1. Ram, and John P. Riggs; and17,832, filed March 9, 1970 .of Charles M. Clarke, Michael 1. Ram, andArnold J. Rosenthal (now U.S. Pat. No. 3,775,520). Each of thesedisclosures is herein incorporated by reference.

In accordance with a particularly preferred carbonization andgraphitization technique a continuous length of stabilized acrylicfibrous material which is non-buming when subjected to an ordinary matchflame and derived from an acrylic fibrous material selected from thegroup consisting of an acrylonitrile homopolymer and acrylonitrilecopolymers which contain at least about 85 mol per cent of acrylonitrileunits and up to about 15 mol per cent of one or more monovinyl unitscopolymerized therewith is converted to a graphitic fibrous materialwhile preserving the original fibrous configuration essentially intactwhile passing containing an inert gaseous atmosphere and a temperaturegradient in which the fibrous material is raised within a period ofabout 20 to about 300 seconds from about 800C. to a temperature of about1,600C. to form a continuous length of carbonized fibrous material, andin which the carbonized fibrous material is subsequently raised fromabout 1,600C. to a maximum temperature of at least about 2,400C. withina period of about 3 to 300 seconds where it is maintained for aboutseconds to about 200 seconds to form a continuous length of graphiticfibrous material.

The equipment utilized to produce the heating zone used to produce thegraphitized carbonaceous starting material may be varied as will beapparent to those skilled in the art. It is essential that the apparatusselected be capable of producing the required temperature whileexcluding the presence of an oxidizing atmosphere.

In a preferred technique the continuous length of fibrous materialundergoing carbonization and graphitization is heated by use of aninduction furnace. In such a procedure the fibrous material may bepassed in the direction of its length through a hollow graphitetube orother susceptor which is situated within the windings of an inductioncoil. By varying the length of the graphite tube, the length of theinduction coil, and the rate at which the fibrous material is passedthrough the graphite tube, many apparatus arrangements capable ofproducing carbonization and graphitization may be selected. For largescale production, it isof course preferred that relatively long tubes orsusceptors be used so that the fibrous material may be passed throughthe same at a more rapid rate while being carbonized and graphitized.The temperature gradient of a given apparatus may be determined byconventional optical pyrometer measurements as will be apparent to thoseskilled in the art. The fibrous material because of its small mass andrelatively large surface area instantaneously assumes substantially thesame temperature as that of the carbonization/graphitization heatingzone through which it is continuously passed.

The Surface Modification The graphitic carbonaceous fibrous material iscoated with a film of a dihydropyridacene polymer which is substantiallyfree of inter-molecular crosslinking consisting of 85 to 100 per cent ofacrylonitrile units wherein the pendant nitrile groups thereof are atleast about 90 per cent cyclized (preferably fully cyclized), and 0 tomol percent of copolymerized monovinyl units. Representative monovinylunits include sytrene, methyl acrylate, methyl methacrylate, vinylacetate, vinyl chloride, vinylidene chloride, vinyl pyridine, and thelike, or a plurality of such comonomers. In a preferred embodiment ofthe invention the dihydropyridacene polymer is substantially free ofintar-molecular cross-linking and consists of 100 mol per cent ofacrylonitrile units wherein the pendant nitrile groups thereof are fullycyclized.

The dihydropyridacene polymer may be derived from 1 an acrylonitrilehomopolymer, or (2) an acrylonitrile copolymer containing at least about85 mol per cent of acrylonitrile units and up to about 15 mol per centof one or more monovinyl units copolymerized therewith. Preferreddihydropyridacene polymers are derived from an acrylonitrilehomopolymer, or from acrylonitrile copolymers which contain at leastabout mol per cent of acrylonitrile units and up to about 5 mol per centof one or more monovinyl units copolymerized therewith. Preferreddihydropyridacene polymer derived from an acrylonitrile homopolymerconsists of recurring units of the structural formula indi cated belowwhere (I) represents and recurring structure of the acrylonitrilehomopolymer (e.g., 4 acrylonitrile units), and (II) represents thestructure of the fully cyclized dihydropyridacene polymer.

(1) CHa CH1 When dihydropyridacene copolymers are formed containing 0 to15 mol per cent of copolymerized monovinyl units the structural formulaof the polymer is directly analogous to that of (II) with the exceptionthat the monovinyl units are randomly dispersed within the polymerchain.

The dihydropyridacene polymer utilized in the process may be formed inaccordance with the procedures described in commonly assigned U.S. Ser.Nos. 88,487 (now U.S. Pat. No. 3,736,309), and 88,489 (now U.S. Pat. No.3,736,310), filed Nov. 10-, 1970, of Klaus H. Gump and Dagobert E.Stuetz which are herein incorporated by reference. More specifically, asdescribed in U.S. Ser. No. 88,487, an acrylic polymer selected from thegroup consisting of anacrylonitrile homopolymer and acrylonitrilecopolymers containing at least about 85 mol per cent of acrylonitrileunits and up to about 15 mol per cent of monovinyl units copolymerizedtherewith may be converted to the desired dihydropyridacene polymer inthe absence of intermolecular cross-linking by (a) providing a solutionof said acrylic polymer which contains: a catalytic quantity of anorganic cyclization promoting agent selected from the group consistingof a carboxylic acid, a sulfonic acid, and a phenol, (b) heating thesolution while present in an essentially oxygen-free zone at atemperature of about to 240C. for about 45 minutes to 16 hours, and (c)recovering the resulting cyclized dihy dropyridacene polymer.Alternatively, as described in U.S. Ser. No. 88,489, an acrylic polymerselected from the group consisting of an acrylonitrile homopolymer andacrylonitrile copolymers containing at least about 85 mol per cent ofacrylonitrile units and up to about 15. mol per cent of monovinyl unitscopolymerized therewith may be converted to a cyclized dihydropyridacenepolymer in the absence of intermolecular cross-linking by (a) providinga mixture of the acrylic polymer and 2-pyrrolidinone, (b) heating themixture while present in an essentially oxygen-free zone at atemperature of about 130 to 220C. for about 2 minutes to 16 hours, and(c) recovering the resulting cyclized dihydropyridacene polymer.

The film of dihydropyridacene polymer is preferably applied to thegraphitic fibrous material by contact with a solution containing thepolymer dissolved in the solvent incapable of destroying the originalfibrous configuration of the fibrous material, and the solventevaporated. Contact may be conveniently carried out by immersing thefibrous material in the solution. Alternatively, contact may be made byspraying, etc. Preferred solvents for the dihydropyridacene polymer areformic acid, sulfuric acid, and polyphosphoric acid. Otherrepresentative solvents are trifluoroacetic acid, or mixtures of theforegoing acids with acetonitrile, methanol, acetone, ethylene glycol,or 2-pyrrolidone (e.g., equal parts by weight solvent mixtures). Whenthe pendant nitrile groups of the dihydropyridacene polymer are notfully cyclized, then more common acrylic solvents, such asN,N-dimethylacetamide may be selected. The dihydropyridacene polymer maybe provided in the solution in a concentration of about 0.01 to 10 percent by weight based upon the weight of the total solution, andpreferably in a concentration of about 0.1 to 5 per cent by weight. Thesolution is preferably at a temperature of about to 50C. when contactedwith the graphitic fibrous material. Evaporation of the solvent from thesolution of dihydropyridacene polymer in contact with the fibrousmaterial is preferably conducted in any convenient manner, such as byheating in air at about 30 to 100C. for a few minutes. The film of thedihydropyridacene polymer present upon the surface of the graphiticfibrous material preferably has a thickness of about 5 to 1,000angstroms, and most preferably a thickness of about 100 to 500angstroms.

The film of dihydropyridacene polymer present upon the surface of thegraphitic carbonaceous fibrous material is provided in intimateassociation with a hydrolyzable organotitanium compound capable ofyielding titanium dioxide upon hydrolysis. The hydrolyzableorganotitanium compound is preferably also applied to the surface of thegraphitic carbonaceous fibrous material as a film in a discrete coatingstep subsequent to the application of the film of dihydropyridacenepolymer. The dihydropyridacene polymer coated fibrous material may becontacted with a solution containing the hydrolyzable organotitaniumcompound dissolved in a solvent incapable of destroying the originalfibrous configuration of the graphitic carbonaceous fibrous material ordeleteriously influencing the film of dihydropyridacene polymer, and thesolvent evaporated.

The chemical structure of the hydrolyzable organotitanium compoundselected for use in the present invention may be varied widely as willbe apparent to those skilled in the chemistry of organotitaniumcompounds. For instance, representative classes of hydrolyzableorganotitanium compounds include the simple alkyl titanates. polymerizedalkyl titanates, alkyl titanate chelates, polymeric titaniumphosphinates, etc.

The alkyl titanates suitable for use in the present process possess atleast one alkyl group having one to eight carbon atoms. The alkyl groupmay optionally be of the cycloalkyl type. The inclusion in the alkyltitanate of at least one alkyl group having one to eight carbon atomsimparts to the organotitanium compounds the capacity to readily undergohydolysis upon exposure to water, such as water vapor present in air. Ina preferred embodiment of the invention the alkyl titanate utilized is atetraalkyl titanate having one to eight carbon atoms in each alkylgroup. Such tetraalkyl titanates commonly possess a formula of Ti(OR).,where R is an alkyl group containing one to eight carbon atoms.Illustrative examples of hydrolyzable tetraalkyl titanates possessing asymmetrical molecular configuration include: tetramethyl titanate,tetraethyl titanate, tetrapropyl tita nate, tetraisopropyl titanate,tetra-n-butyl titanate, tetraisobutyl titanate, tetrapentyl titanate,tetra(2- ethylhexyl) titanate, tetraoctyl titanate, and the like. Thelower alkyl titanates containing one to four carbon atoms per alkylgroup are particularly preferred. Such compounds are commonly lightyellow liquids. Mixed tetraalkyl titanates may also be utilized in whichat least a portion of the alkyl groups of each molecule exceed eightcarbon atoms in length. For instance, compounds such as isopropylstearyltitanate may be employed. Representative commercially available simplealkyl titanates are Tyzor TPl", Tyzor TBT, Tyzor TOT, and Tyzor APorganic titanates which are provided by the Du Pont C0.

It is also possible for the alkyl titanates discussed above to bepartially condensed or polymerized to form relatively low molecularweight polytitanates. As is well known in the chemistry oforganotitanium compounds, such condensation or polymerization productsmay result from the reaction of the alkyl titanate with less than thestoichiometric amount of water. Condensed esters of varying degrees ofhydrolysis from hexaalkoxy dititanates, [(RO) Ti] O, to dialkoxypolytitanates, RO[- Ti(OR) O-],R, can be formed by the addition of therequired amount of water. See U.S. Pat. No. 2,689,858. For example,hexaisopropyl dititanate, or hexabutyl dititanate possessing Istructural configurations of (C H O) Ti-O-Ti(OC l-l and (C H O)Ti-O-Ti(OC l-l respectively, may be selected. A representativecommercially available polymerized alkyl titanate is a polymerized butyltitanate designated as Tyzor PB organic titanate which is provided bythe Du Pont Co.

Alkyl titanate chelates may be formed by reacting either a beta-diketone(e.g., acetylacetone) or a ketoester (e.g., ethyl acetoacetate, diethylmalonate, and malononitrile) with an alkyl titanate having two to fourcarbon atoms in each alkyl group (e.g., tetraethyl titanate, tetrapropyltitanate, tetraisopropyl titanate, tetran-butyl titanate, andtetraisobutyl titanate). A preferred chelating agent is acetylacetone,and a preferred alkyl titanate coreactant is tetraisopropyl titanate.The titanium chelates may be formed by simply mixing the chelating agentwith the alkyl titanate coreactant in a mol ratio of l to 4 mols ofchelating agent per 1 mol of the alkyl titanate. During the reaction,which is exothermic, a proportionate number of the alkoxy groups of thealkyl titanate are replaced, and may be recovered if desired as thecorresponding alcohol by suitable distillation techniques, such as bydistillation at relatively low temperatures under reduced pressure. Ifmore drastic distillation procedures are attempted, polymeric insolublecondensation products tend to form. Distillation maybe terminated whenthe stoichiometric quantity of alcohol is recovered. Particularlypreferred organotitanium reaction products for use in the presentinvention are prepared by reacting approximately 2 mols of chelatingagent per 1 mol of the alkyl titanate, so that approximately one-half ofthe alkoxy groups are replaced on each alkyl titanate molecule. Forinstance, if acetylacetone and tetraisopropyl titanate are thecoreactants, the reaction product is believed to be largelydi-isopropoxytitanium bis-(acetyl acetonate). A representativecommercially available titanium chelate formed by the use of anacetylacetone chelating agent is Tyzor AA organic titanate which isprovided by the Du Pont Co.

Polymeric titanium phosphinates may be formed as described by B. P.Block in Inorganic Macromolecules Reviews, Vol. 1, Pages ll-l25 (1970).The preferred polymeric titanium phosphinate is poly(bis-diphenylphosphenyl) titanate having recurring units of the for mula The solventutilized in the formation of the solution containing the hydrolyzableorganotitanium compound may be varied widely as will be apparent tothose skilled in the art. It is essential, however, that the solvent beincapable of destroying the original fibrous configuration of thegraphitic carbonaceous fibrous .material or otherwise adverselyinfluencing its properties or the properties of the dihydropyridacenepolymer film. Additionally, the solvent must be incapable of producingany substantial hydrolysis of the organotitanium compound dissolvedtherein. The particular organotitanium compound utilized may influencethe solvent which is selected. Representativesolvents may be selectedfrom the following: benzene, carbon tetrachloride, isopropyl alcohol,n-butyl alcohol, nheptane. octane, trichloroethylene, dioxane, petroleumether, xylol. and the like. in addition when the hydrolyzableorganotitanium compound is an alkyl titanate chelate the solvent mayeven be water which is adjusted to a pH of about 3 (e.g., with aceticacid).

The hydrolyzable organotitanium compound is provided in the solution ina concentration of about 0.] to 5 per cent by weight based upon thetotal weight of the solution, and preferably in a concentration of about0.1 to l per cent by weight based upon the total weight of the solution.The solution is preferably provided at a temperature of about 0 to 50C.when contacted with the graphitic carbonaceous fibrous material, andmost preferably at a temperature of about to C. Contact of the graphiticcarbonaceous fibrous material with the solution may be accomplished on abatch or a continuous basis. For instance, the fibrous material may bewound upon a support and immersed in the solution. A continuous lengthof the fibrous material may be conveniently passed on a continuous basisthrough a vessel containing the solution. Alternatively, the stabilizedacrylic fibrous material may be sprayed with the solution. When contactis made via immersion in the solution, residence times of about 1 to 10seconds may be. conveniently utilized. The temperature of the solutionand the duration of the contact are generally not critical to theoperation of the process. The concentration of the organotitaniumcompound in the solution will influence the thickness of the coatingachieved upon evaporation of the solvent.

The solvent of the solution in contact with the fibrous material is nextevaporated so that a film of the hydrolyzable organotitanium compoundisdeposited upon the surface of the fibrous material. Evaporation of thesolvent may be conducted in a circulating gaseous atmosphere, e.g.. air.The film is preferably substantially uniform and provided in a thicknessof about 4 to 200 angstroms, and most preferably in a thickness of about4 to 40 angstroms.

The film of hydrolyzable organotitanium compound isnext hydrolyzed toform a corresponding film of titanium dioxide upon the surface of thefibrous material. The film of titanium dioxide is preferably provided ina thickness of about 4 to 200 angstroms, and most preferably in athickness of about 4 r040 angstroms. The

hydrolysis is preferably carried out by heating in a gaseous atmospherewhich contains water vapor until the hydrolysis reaction issubstantially complete. The period of time required to complete thehydrolysis reaction varies with the specific organotitanium compoundinvolved, the temperature of the gaseous atmosphere, the thickness ofthe film, and the concentration of water vapor in the gaseousatmosphere. If desired the evaporation step described above may beconducted in the same zone in which hydrolysis is carried out.Hydrolysis treatment times employing air (e.g., of about 5 to 100 percent relative humidity) at about 5 to 400C. commonly range from about0.1 to 60 minutes. Preferred hydrolysis reactions utilize air of about 5to 60 per cent relative humidity at about 20 to 300C. for a treatmenttime of about 0.25 to 60 minutes. Alternatively, the hydrolysis of thefilm of organotitanium compound may be similarly accomplished by contactwith a strong acid bath,by contact with steam, etc. Hyrolysis carriedout in air at about 200 to 300C. additionally serves to oxidativelycross-link the dihy' dropyridacene polymer and thereby to furtherenhance its thermal stability.

The resulting fibrous material bearing a film of dihydropyridacenepolymer in intimate association with titanium dioxide upon its surfaceis heated in an inert (i.e. non-oxidizing)gaseous atmosphere at atemperature of at least about 900C, but not exceeding l,800C.. until thedihydropyridacene polymer portion of the film is carbonized to apredominantly amorphous carbon form containing at least about per centcarbonby weight is formed (preferably at least per cent carbon byweight) to produce a. predominantly graphitic carbonaceous fibrousmaterial which exhibits an enhanced ability to bond to a matrixmaterial. Suitable inert gaseous atmospheres include nitrogen, argon,and helium. At temperatures much below about 900C. the carbonizationreaction is inordinately slow. At processing temperatures much aboveabout l,800C. a fiber is produced exhibiting no substantial enhancementin its ability to bond to a matrix material as a result of theintermediate processing heretofore described. The. maximum carbonizationtemperature is preferably about l,200C. A carbonization temperaturegradient optionally may be employed wherein the fibrous material isheated at gradually increasing temperatures above 900 C. up to aboutl,800C. (preferably up to about l,200C.). The carbonization reaction maybe carried out in accordance with known techniques on either a batch ora continuous basis in an. inert gaseousatmosphere provided the maximumtemperature of about l,800C. is not exceeded. carbonization residencetimes at the temperatures indicated commonly range from about I to 10minutes. If desired, considerably longer carbonization residence timesmay be selected.

In a preferred technique a continuous length of the coated graphiticcarbonaceous fibous material undergoing carbonization is heated by useof an induction furnace. In such a procedure the fibrous material may bepassed in the direction of its length through a hollow graphite tube orother susceptor which is situated within the windings of an inductioncoil. By varying the length of the graphite tube, the length of theinduction coil, and the rate at which the fibrous material is passedthrough the graphite tube, many apparatus arrangements capable ofproducing carbonization may be selected. For large scale production, itis of course preferred that relatively long tubes or susceptors be usedso that the fibrous material may be passed through the same at a morerapid rate while the dihydropyridacene polymer is carbonized. Thetemperature gradient of a given apparatus may be determined byconventional optical pyrometer measurements as will be apparent to thoseskilled in the art. The fibrous material because of its small mass, andrelatively large surface area instantaneously assumes essentially thesame temperature as that of the zone through which it is continuouslypassed. Resistance heated carbonization heating zones may also beutilized in another preferred embodiment of the process.

The theory whereby the surface of the resulting graphitic carbonaceousfibers are rendered capable of enhanced adhesion to a matrix material isconsidered complex and incapable of simple explanation. It is believed,however, that the improved bonding characteristics can be traced to thepresence of a substantially uniform film of predominantly amorphouscarbon and titanium metal in intimate admixture (e.g. of to 1,000angstroms and preferably 100 to 500 angstroms thickness) upon thesurface of the resulting graphitic carbon fibers.

The surface modification of the present process exhibits an appreciableshelf life, and is not diminished to any substantial degree upon thepassage of several weeks, or more.

The process of the present invention facilitates improved adhesivebonding between the predominantly graphitic carbonaceous fibers and amatrix material which may be either resinous (e.g., a thermosettingresinous material), or metallic. The composite articles of the resultinginvention may be formed by conventional composite formation techniques.The resulting fiber reinforced composites which incorporate the graphitefibers of the present invention exhibit an enhanced interlaminar shearstrength. Also, other composite properties such as flexural strength,compressive strength, etc., may be enhanced. The resinous matrixmaterial employed in the formation of such composite materials iscommonly a polar thermosetting resin such as an epoxy, a polyamide, apolyester, a phenolic, etc. The metallic matrix material employed in theformation of such composite materials may be aluminum, titanium,

-chromium, nickel, copper, silver, steel, etc.

The following examples are given as specific illustrations of theinvention. It should be understood, however, that the invention is notlimited to the specific de tails set forth in the examples.

' EXAMPLE I The graphite carbonaceous yarn undergoing treatment wasderived from acrylonitrile homopolymer yarn in accordance withprocedures described in commonly assigned U.S. Ser. Nos. 749,957 (nowabandoned), filed Aug. 5, 1968 and 777,275 (now abandoned) filed Nov.20, 1968. The maximum graphitization temperature experienced by the yarnwas 2,200C. The yarn consisted of a 9,500 fil bundle having a totaldenier of about 7,600, had a carbon content in excess of 99 per cent byweight, exhibited a predominantly graphitic x-ray diffraction pattern, asingle filamenttenacity of about 13 grams per denier, and a singlefilament Youngs modulus of about 50 million psi.

The graphitic carbonaceous yarn was continuously passed through a 1.0per cent by weight formic acid solution of dihydropyridacene polymerwhich was substantially free of inter-molecular cross-linking consistingof 100 mol per cent of acrylonitrile units wherein the pendant nitrilegroups thereof were fully cyclized. The dihydropyridacene polymer wasformed in accordance with the procedure described in commonly assignedU.S. Ser. No. 88,487 (now US. Pat. No. 3,736,309), filed Nov. 10, 1970of Klaus H. Gump and Dagobert E. Stuetz. The yarn was immersed in thedihydropyridacene polymer solution provided at about 25 C. for about 10seconds.

The formic acid solvent in contact with the yarn was evaporated byheating in air at C. for 30 minutes to form a substantially uniform filmof dihydropyridacene polymer having a thickness of about angstroms, uponthe surface of the graphitic carbonaceous fibrous material.

The resulting yarn was continuously passed through a 5.0 per cent byweight benzene solution of a polymerized alkyl titanate provided atabout 25C. for about 10 seconds. More specifically, the organo-titaniumcompound was polymerized tetrabutyl titanate of the approximate formulaC H O[Ti(OC l-l O] C H commercially available from the Du Pont Companyunder the designation Tyzor PB organic titanate.

The benzene solvent'was evaporated from the solution in contact with theyarn and the film of polymerized tetrabutyl titanate present on thesurface of the yarn was simultaneously hydrolyzed by heating in air at270C. having a relative humidity of about 5 per cent for minutes. Asubstantially uniform film of titanium dioxide having a thickness ofabout l00 angstroms was formed upon the surface of the graphiticcarbonaceous yarn in intimate association with the film ofdihydropyridacene polymer. The air heat treatment at 270C. also servedto enhance the thermal stability of the dihydropyridacene polymer and tooxidatively cross-link the same.

The dihydropyridacene polymer film present upon the fibrous material wascarbonized to a predominantly amorphous carbon form by continuouspassage of the yarn through a muffle furnace provided with a nitrogenatmosphere having a temperature gradient to produce a continuous lengthof carbon fiber containing in excess of 95 per cent carbon by weight.While passing through the furnace the yarn was raised from roomtemperature (i.e. 25C.) 'to l,200C. in about seconds where it wasmaintained for about 75 seconds. Present upon the surface of theresulting predominantly graphitic carbonaceous yarn was a filmconsisting of predominantly amorphous carbon and titanium metal having atotal thickness of about 200 angstroms.

P16. 1 is a photograph made with the aid of'a scanning electronmicroscope of the resulting predominantly graphitic carbonaceous fiberat a magnification of 6600X. FIG. 2 is a photograph made with the aid ofa scanning electron microscope a control predominantly graphiticcarbonaceous fiber at a magnification of 6400X which underwent no formof surface modification. Electrochemical surface area measurementsindicated that the fiber of P16. 1 exhibited a ten fold surface areaincrease over the fiber of FIG. 2.

A composite article was next formed employing the surface modified yarnsample as a reinforcing medium in a resinous matrix. The compositionarticle was a rectangular bar consisting of about 50 per cent by volumeof the yarn and having dimensions of /s inch X A inch X inches. Thecomposite article was formed by impregnation of the yarn in a liquidepoxy resinhardener mixture at 50C. followed by unidrectional layup ofthe required quantity of the impregnated yarn in a steel mold andcompression molding of the layup for 2 hours at 93C., and 2.5 hours at200C. in a heated platen press at about 100 psi pressure. The mold wascooled slowly to room temperature, and the composite article was removedfrom the mold cavity and cut to size for testing. The resinous matrixmaterial used in the formation of the composite article was provided asa solventless system which contained 100 parts by weight of epoxy resinand 98 parts by weight of anhydride curing agent. t

The horizontal interlaminar. shear strength of the composite article wasdetermined by short beam testing of the fiber reinforced compositeaccording to the pro cedure of ASTM D2344-65T as modified for straightbar testing at a 4:1 span to depth ratio and was found to be 8,815 psi.

For comparative purposes a composite article was formed as heretoforedescribed employing the control predominantly graphitic carbonaceousyarn which was not subjected to any form of surface modification. The

T average horizontal interlaminar shear strength of the compositearticle was only 2,800 psi.

[n a comparative surface modification procedure wherein Example 1 wasrepeated with the exception that no organotitanium compound was appliedto the yarn, the resulting composite article failed to exhibit asubstantially improved interlaminar shear strength.

In a comparative surface modification procedure wherein Example I wasrepeated with the exception that no dihydropyridacene polymer wasapplied to the yarn, the resulting composite article failed to exhibit asubstantially improved interlaminar shear strength.

EXAMPLE ll Example 1 is repeated with the exception that the yarn isimmersed in a 0.1 per cent by weight formic acid solution ofdihydropyridacene polymer, and following evaporation of the solvent in a0.5 per cent by weight benzene solution of the polymerized tetrabutyltitanate.

The horizontal interlaminar shear strength of the resulting compositearticle is found to be 10,410 psi.

EXAMPLE 111 Example 1 is repeated with the exception that thehydrolyzable organotitanium compound utilized is an alkyl titanatechelate of the titanium acetylacetonate chemical type which is appliedfrom a 1.0 per cent by weight aqueous solution wherein the pH of thesolution is adjusted to about 3 by the presence of acetic acid. Thealkyl titanate chelate is commercially available from the Du PontCompany under the designation of Tyzor AA organic titanate.

The resulting composite article exhibits a substantially enhancedhorizontal interlaminar shear strength when compared with the control.

EXAMPLE lV Example I is repeated with theexception that the hy- EXAMPLEV Example I is repeated with the exception that the hydrolyzableorganotitanium compound utilized is a simple alkyl titanate. Morespecifically, the organotitanium compound is tetraisopropyltitanatecommercially available from the Du Pont Company under the designationTyzor TPT. The tetraalkyl titanate is applied from a 5 per cent byweight solution inisopropanol.

The resulting composite article exhibits a substantially enhancedhorizontal interlaminar shear strength when compared with the control.

Although the invention has been described with preferred embodiments, itis to be understood that variations and modifications may be resorted toas will be apparent to those skilled in the art. Such variations andmodifications are to be considered within the purview and scope of theclaims appended hereto.

We claim:

l. A process for the surface modification of a predominantly graphiticcarbonaceous fibrous material containing at least 90 per cent carbon byweight comprising: p

a. coating said fibrous material with a film of a dihydropyridacenepolymer which is substantially free of inter-molecular cross-linkingconsisting of to 100 mol per cent of acrylonitrile units wherein thependant nitrile groups thereof are at least about per cent cyclized, and0 to 15 mol per centof co polymerized monovinyl units, with saiddihydropyridacene polymer film being in intimate association with ahydrolyzable organotitanium compound capable of yielding titaniumdioxide upon hydrolysis,

b. hydrolyzing said hydrolyzable organotitanium compound in intimateassociation with said dihydroyridacene polymer film to form titaniumdioxide. and

c. carbonizing said dihydropyridacene polymer portion of said filmpresent upon said fibrous material to a predominantly amorphous carbonform containing at least about 90 per cent by weight by heating in aninert gaseous atmosphere at a temperature of at least about 900C, butnot exceeding about l,800C. to produce :a predominantly graphiticcarbonaceous fibrous material which possesses an enhanced ability tobond to a matrix material.

2. A process according to claim 1 wherein said dihydropyridacene polymerconsists of mol per cent of acrylonitrile units wherein the pendantnitrile groups thereof are fully cyclized.

3. An improved process according to claim 1 wherein said hydrolyzableorganotitanium compound capable of yielding titanium dioxide uponhydrolysis is selected from the group consisting of the simple alkyltitanates, the polymerized alkyl titanates, the alkyl titanate chelates,and the polymeric titanium phosphinates.

4. An improved process according to claim 1 wherein said hydrolyzableorganotitanium compound in intimate association with saiddihydropyridacene film is hydrolyzed by heating in air.

5. An improved process according to claim 1 wherein said inert gaseousatmosphere is provided at a temperature of at least about 900C, but notexceeding about 6. An improved process according to claim 1 wherein saidinert gaseous atmosphere is selected from the group consisting ofnitrogen, argon, and helium.

7. A process for the surface modification of a predominantly graphiticcarbonaceous fibrous material containing at least 95 per cent carbon byweight comprising:

a. contacting said fibrous material with a solution containing adihydropyridacene polymer which is substantially free of inter-molecularcross-linking consisting of 85 to l mol per cent of acrylonitrile unitswherein the pendant nitrile groups thereof are fully cyclized, and 0 to15 mol per cent of copolymerized monovinyl units which is dissolved in asolvent incapable of destroying the original fibrous configuration ofsaid fibrous material,

. evaporating said solvent of said solution in contact with said fibrousmaterial whereby a substantially uniform first film of saiddihydropyridacene polymer having a thickness of about 5 to 1,000angstroms is deposited upon the surface of said fibrous material,

c. contacting said resulting fibrous material bearing said first film ofsaid dihydropyridacene polymer upon the surface thereof with a solutioncontaining a hydrolyzable organotitanium compound selected from thegroup consisting of the simple alkyl titanates, the polymerized alkyltitanates, the alkyl titanate chelates, and the polymeric titaniumphosphinates capable of yielding titanium dioxide upon hydrolysis whichis dissolved in a solvent incapable of dissolving said first polymerfilm or destroying the original fibrous configuration of said fibrousmaterial,

evaporating said solvent of said solution in contact with said fibrousmaterial whereby a substantially uniform second film of saidhydrolyzable organotitanium compound having a thickness of about 4 to200 angstroms is deposited upon the surface of said fibrous material,

e. hydrolyzing said second film of said hydrolyzable organotitaniumcompound present upon the surface of said fibrous material by heating inair at about 200 to 300C. to form a substantially uniform film oftitanium dioxide having a thickness of about 4 to 200 angstroms uponsaid fibrous material in intimate associated with said first film ofsaid dihydropyridacene polymer while said first film ofdihydropyridacene polymer is simultaneously oxidatively cross-linked,and

f. carbonizing said first film of dihydropyridacene polymer film presentupon said fibrous material to an amorphous carbon form containing atleast about per cent carbon by weight by heating in an inert gaseousatmosphere selected from the group consisting of nitrogen, argon, andhelium at a temperature of at least about 900C, but not exceeding aboutl,200C. to produce a predominantly graphitic carbonaceous fibrousmaterial which possesses an enhanced ability to bond to a matrixmaterial.

8. A process according to claim 7: wherein said dihydropyridacenepolymer consists of mol per cent of acrylonitrile units wherein thependant nitrile groups thereof are fully cyclized.

9. A process according to claim 7 wherein said dihydropyridacene polymeris dissolved in a solvent selected from the group consisting of formicacid, sulfuric acid, and polyphosphoric acid.

10. A process according to claim 7 wherein said hydrolyzableorganotitanium compound is a polymerized tetrabutyltitanate.

11. A process according to claim 7 wherein said by drolyzableorganotitanium compound is titanium acetylacetonate.

12. An improved process according to claim 7 wherein said hydrolyzableorganotitanium compound is a simple tetraalkyl titanate having 1 to 8carbon atoms in each alkyl groupf I 13. A process according to claim 12wherein said hydrolyzable organotitanium compound istetraisopropyltitanate. I

14. A composite article exhibiting an enhanced interlaminar shearstrength comprising a resinous matrix material having incorporatedtherein a predominantly graphitic carbonaceous fibrous materialcontaining at least about 90 percent carbon by weight having a film ofpredominantly amorphous carbon containing at least about 90 percentcarbon by weight upon the surface thereon in intimate association withmetallic titanium formed in accordance with the process of claim 1, withsaid film of predominantly amorphous carbon in intimate association withmetallic titanium having a thickness of about 10 to 1.000 angstroms.

2. A process according to claim 1 wherein said dihydropyridacene polymerconsists of 100 mol per cent of acrylonitrile units wherein the pendantnitrile groups thereof are fully cyclized.
 3. An improved processaccording to claim 1 wherein said hydrolyzable organotitanium compoundcapable of yielding titanium dioxide upon hydrolysis is selected fromthe group consisting of the simple alkyl titanates, the polymerizedalkyl titanates, the alkyl titanate chelates, and the polymeric titaniumphosphinates.
 4. An improved process according to claim 1 wherein saidhydrolyzable organotitanium compound in intimate association with saiddihydropyridacene film is hydrolyzed by heating in air.
 5. An improvedprocess according to claim 1 wherein said inert gaseous atmosphere isprovided at a temperature of at least about 900*C., but not exceedingabout 1,200*C.
 6. An improved process according to claim 1 wherein saidinert gaseous atmosphere is selected from the group consisting ofnitrogen, argon, and helium.
 7. A process for the surface modificationof a predominantly graphitic carbonaceous fibrous material containing atleast 95 per cent carbon by weight comprising: a. contacting saidfibrous material with a solution containing a dihydropyridacene polymerwhich is substantially free of inter-molecular cross-linking consistingof 85 to 100 mol per cent of acrylonitrile units wherein the pendantnitrile groups thereof are fully cyclized, and 0 to 15 mol per cent ofcopolymerized monovinyl units which is dissolved in a solvent incapableof destroying the original fibrous configuration of said fibrousmaterial, b. evaporating said solvent of said solution in contact withsaid fibrous material whereby a substantially uniform first film of saiddihydropyridacene polymer having a thickness of about 5 to 1,000angstroms is deposited upon the surface of said fibrous material, c.contacting said resulting fibrous material bearing said first film ofsaid dihydropyridacene polymer upon the surface thereof with a solutioncontaining a hydrolyzable organotitanium compound selected from thegroup consisting of the simple alkyl titanates, the polymerized alkyltitanates, the alkyl titanate chelates, and the polymeric titaniumphosphinates capable of yielding titanium dioxide upon hydrolysis whichis dissolved in a solvent incapable of dissolving said first polymerfilm or destroying the original fibrous configuration of said fibrousmaterial, d. evaporating Said solvent of said solution in contact withsaid fibrous material whereby a substantially uniform second film ofsaid hydrolyzable organotitanium compound having a thickness of about 4to 200 angstroms is deposited upon the surface of said fibrous material,e. hydrolyzing said second film of said hydrolyzable organotitaniumcompound present upon the surface of said fibrous material by heating inair at about 200* to 300*C. to form a substantially uniform film oftitanium dioxide having a thickness of about 4 to 200 angstroms uponsaid fibrous material in intimate associated with said first film ofsaid dihydropyridacene polymer while said first film ofdihydropyridacene polymer is simultaneously oxidatively cross-linked,and f. carbonizing said first film of dihydropyridacene polymer filmpresent upon said fibrous material to an amorphous carbon formcontaining at least about 95 per cent carbon by weight by heating in aninert gaseous atmosphere selected from the group consisting of nitrogen,argon, and helium at a temperature of at least about 900*C., but notexceeding about 1,200*C. to produce a predominantly graphiticcarbonaceous fibrous material which possesses an enhanced ability tobond to a matrix material.
 8. A process according to claim 7 whereinsaid dihydropyridacene polymer consists of 100 mol per cent ofacrylonitrile units wherein the pendant nitrile groups thereof are fullycyclized.
 9. A process according to claim 7 wherein saiddihydropyridacene polymer is dissolved in a solvent selected from thegroup consisting of formic acid, sulfuric acid, and polyphosphoric acid.10. A process according to claim 7 wherein said hydrolyzableorganotitanium compound is a polymerized tetrabutyltitanate.
 11. Aprocess according to claim 7 wherein said hydrolyzable organotitaniumcompound is titanium acetylacetonate.
 12. An improved process accordingto claim 7 wherein said hydrolyzable organotitanium compound is a simpletetraalkyl titanate having 1 to 8 carbon atoms in each alkyl group. 13.A process according to claim 12 wherein said hydrolyzable organotitaniumcompound is tetraisopropyltitanate.
 14. A composite article exhibitingan enhanced interlaminar shear strength comprising a resinous matrixmaterial having incorporated therein a predominantly graphiticcarbonaceous fibrous material containing at least about 90 percentcarbon by weight having a film of predominantly amorphous carboncontaining at least about 90 percent carbon by weight upon the surfacethereon in intimate association with metallic titanium formed inaccordance with the process of claim 1, with said film of predominantlyamorphous carbon in intimate association with metallic titanium having athickness of about 10 to 1,000 angstroms.